Mosquito midguts and malaria: cell biology, compartmentalization and immunology

Ingested human insulin inhibits the mosquito NF-κB-dependent immune response to Plasmodium falciparum

We showed previously that ingested human insulin activates the insulin/IGF-1 signaling pathway in Anopheles stephensi and increases the susceptibility of these mosquitoes to Plasmodium falciparum. In other organisms, insulin can alter immune responsiveness through regulation of NF-κB transcription factors, critical elements for innate immunity that are also central to mosquito immunity. We show here that insulin signaling decreased expression of NF-κB-regulated immune genes in mosquito cells stimulated with either bacterial or malarial soluble products. Further, human insulin suppressed mosquito immunity through sustained phosphatidylinositol 3-kinase activation, since inhibition of this pathway led to decreased parasite development in the mosquito. Together, these data demonstrate that activation of the insulin/IGF-1 signaling pathway by ingested human insulin can alter NF-κB-dependent immunity, and ultimately the susceptibility, of mosquitoes to P. falciparum.

Intracellular immune responses of dipteran insects

Vector-borne diseases, transmitted by bloodsucking arthropods, pose worldwide socio-medical and economical problems. Some of the major human infectious diseases, such as malaria, Dengue fever, and yellow fever, are transmitted by mosquitoes. While the majority of pathogens enjoy extracellular life styles in insects, viruses and some endosymbionts are strictly intracellular. Here, we summarize our knowledge on defense reactions against intracellular microorganisms in dipteran insects and discuss the potential of insects as models to study human pathogens.

Mosquito immunity

Throughout their lifetime, mosquitoes are exposed to pathogens during feeding, through breaks in their cuticle and following pathogen-driven cuticular degradation. To resist infection, mosquitoes mount innate cellular and humoral immune responses that are elicited within minutes of exposure and can lead to pathogen death via three broadly defined mechanisms: lysis, melanization and hemocyte-mediated phagocytosis. This chapter reviews our current understanding of the mosquito immune system, with an emphasis on the physical barriers that prevent pathogens from entering the body, the organs and tissues that regulate immune responses and the mechanistic and molecular bases of immunity.

Epidemiology and infectivity of Plasmodium falciparum and Plasmodium vivax gametocytes in relation to malaria control and elimination

Malaria remains a major cause of morbidity and mortality in the tropics, with Plasmodium falciparum responsible for the majority of the disease burden and P. vivax being the geographically most widely distributed cause of malaria. Gametocytes are the sexual-stage parasites that infect Anopheles mosquitoes and mediate the onward transmission of the disease. Gametocytes are poorly studied despite this crucial role, but with a recent resurgence of interest in malaria elimination, the study of gametocytes is in vogue. This review highlights the current state of knowledge with regard to the development and longevity of P. falciparum and P. vivax gametocytes in the human host and the factors influencing their distribution within endemic populations. The evidence for immune responses, antimalarial drugs, and drug resistance influencing infectiousness to mosquitoes is reviewed. We discuss how the application of molecular techniques has led to the identification of submicroscopic gametocyte carriage and to a reassessment of the human infectious reservoir. These components are drawn together to show how control measures that aim to reduce malaria transmission, such as mass drug administration and a transmission-blocking vaccine, might better be deployed.

Human immune responses that reduce the transmission of Plasmodium falciparum in African populations

Malaria-infected individuals can develop antibodies which reduce the infectiousness of Plasmodium gametocytes to biting Anopheles mosquitoes. When ingested in a bloodmeal together with gametocytes, these antibodies reduce or prevent subsequent parasite maturation in the insect host. This transmission-blocking immunity is usually measured in human sera by testing its effect on the infectivity of gametocytes grown in vitro. Here we evaluate evidence of transmission-blocking immunity in eight studies conducted in three African countries. Plasmodium falciparum gametocytes isolated from each individual were fed to mosquitoes in both autologous plasma collected with the parasites, and permissive serum from non-exposed donors. Evidence of transmission reducing effects of autologous plasma was found in all countries. Experiments involving 116 Gambian children (aged 0.5-15 years) were combined to determine which factors were associated with transmission reducing immune responses. The chances of infecting at least one mosquito and the average proportion of infected mosquitoes were negatively associated with recent exposure to gametocytes and sampling late in the season. These results suggest that effective malaria transmission-reducing antibodies do not commonly circulate in African children, and that recent gametocyte carriage is required to initiate and/or boost such responses.

The dynamics of naturally acquired immune responses to Plasmodium falciparum sexual stage antigens Pfs230 & Pfs48/45 in a low endemic area in Tanzania

Background

Naturally acquired immune responses against sexual stages of P. falciparum can reduce the transmission of malaria from humans to mosquitoes. These antigens are candidate transmission-blocking vaccines but little is known about the acquisition of sexual stage immunity after exposure to gametocytes, or their longevity and functionality. We conducted a longitudinal study on functional sexual stage immune responses.

Methodology/Principal Findings

Parasitaemic individuals (n = 116) were recruited at a health centre in Lower Moshi, Tanzania. Patients presented with gametocytes (n = 16), developed circulating gametocytes by day 7 (n = 69) or between day 7 and 14 (n = 10) after treatment or did not develop gametocytes (n = 21). Serum samples were collected on the first day of gametocytaemia and 28 and 84 days post-enrolment (or d7, 28, 84 after enrolment from gametocyte-negative individuals). Antibody responses to sexual stage antigens Pfs230 and Pfs48/45 were detected in 20.7% (72/348) and 15.2% (53/348) of the samples, respectively, and were less prevalent than antibodies against asexual stage antigens MSP-1₁₉ (48.1%; 137/285) and AMA-1 (52.4%; 129/246)(p<0.001). The prevalence of anti-Pfs230 (p = 0.026) and anti-Pfs48/45 antibodies (p = 0.017) increased with longer duration of gametocyte exposure and had an estimated half-life of approximately 3 months. Membrane feeding experiments demonstrated a strong association between the prevalence and concentration of Pfs230 and Pfs48/45 antibodies and transmission reducing activity (TRA, p<0.01).

Conclusions/Significance

In a longitudinal study, anti-Pfs230 and Pf48/45 antibodies developed rapidly after exposure to gametocytes and were strongly associated with transmission-reducing activity. Our data indicate that the extent of antigen exposure is important in eliciting functional transmission-reducing immune responses.

Serine protease-mediated host invasion by the parasitic nematode Steinernema carpocapsae

Steinernema carpocapsae is an insect parasitic nematode used in biological control, which infects insects penetrating by mouth and anus and invading the hemocoelium through the midgut wall. Invasion has been described as a key factor in nematode virulence and suggested to be mediated by proteases. A serine protease cDNA from the parasitic stage was sequenced (sc-sp-1); the recombinant protein was produced in an Escherichia coli system, and a native protein was purified from the secreted products. Both proteins were confirmed by mass spectrometry to be encoded by the sc-sp-1 gene. Sc-SP-1 has a pI of 8.7, a molecular mass of 27.3 kDa, a catalytic efficiency of 22.2 × 10(4) s(-1) m(-1) against N-succinyl-Ala-Ala-Pro-Phe-pNA, and is inhibited by chymostatin (IC 0.07) and PMSF (IC 0.73). Sc-SP-1 belongs to the chymotrypsin family, based on sequence and biochemical analysis. Only the nematode parasitic stage expressed sc-sp-1. These nematodes in the midgut lumen, prepared to invade the insect hemocoelium, expressed higher levels than those already in the hemocoelium. Moreover, parasitic nematode sense insect peritrophic membrane and hemolymph more quickly than they do other tissues, which initiates sc-sp-1 expression. Ex vivo, Sc-SP-1 was able to bind to insect midgut epithelium and to cause cell detachment from basal lamina. In vitro, Sc-SP-1 formed holes in an artificial membrane model (Matrigel), whereas Sc-SP-1 treated with PMSF did not, very likely because it hydrolyzes matrix glycoproteins. These findings highlight the S. carpocapsae-invasive process that is a key step in the parasitism thus opening new perspectives for improving nematode virulence to use in biological control.

Invasion of mosquito salivary glands by malaria parasites: prerequisites and defense strategies

The interplay between vector and pathogen is essential for vector-borne disease transmission. Dissecting the molecular basis of refractoriness of some vectors may pave the way to novel disease control mechanisms. A pathogen often needs to overcome several physical barriers, such as the peritrophic matrix, midgut epithelium and salivary glands. Additionally, the arthropod vector elicites immune responses that can severely limit transmission success. One important step in the transmission of most vector-borne diseases is the entry of the disease agent into the salivary glands of its arthropod vector. The salivary glands of blood-feeding arthropods produce a complex mixture of molecules that facilitate blood feeding by inhibition of the host haemostasis, inflammation and immune reactions. Pathogen entry into salivary glands is a receptor-mediated process, which requires molecules on the surface of the pathogen and salivary gland. In most cases, the nature of these molecules remains unknown. Recent advances in our understanding of malaria parasite entry into mosquito salivary glands strongly suggests that specific carbohydrate molecules on the salivary gland surface function as docking receptors for malaria parasites.

Plasmodium falciparum GPI toxin: a common foe for man and mosquito

The glycosylphosphatidylinositol (GPI) anchor of the malaria parasite, Plasmodium falciparum, which can be regarded as an endotoxin, plays a role in the induced pathology associated with severe malaria in humans. However, it is unclear whether the main mosquito vector, Anopheles gambiae, can specifically recognize, and respond to GPI from the malaria parasite. Recent data suggests that the malaria vector does mount a specific response against malaria GPI. In addition, following the strong immune response, mosquito fecundity is severely affected, resulting in a significant reduction in viable eggs produced. In this mini-review we look at the increased interest in understanding the way that malaria antigens are recognized in the mosquito, and how this relates to a better understanding of the interactions between the malaria parasite and both human and vector.

Evolutionary drivers of parasite-induced changes in insect life-history traits from theory to underlying mechanisms

Many hosts are able to tolerate infection by altering life-history traits that are traded-off one against another. Here the reproductive fitness of insect hosts and vectors is reviewed in the context of theories concerning evolutionary mechanisms driving such alterations. These include the concepts that changes in host reproductive fitness are by-products of infection, parasite manipulations, host adaptations, mafia-like strategies or host compensatory responses. Two models are examined in depth, a tapeworm/beetle association, Hymenolepis diminuta/Tenebrio molitor and malaria infections in anopheline mosquitoes. Parasite-induced impairment of vitellogenesis ultimately leads to a decrease in female reproductive success in both cases, though by different means. Evidence is put forwards for both a manipulator molecule of parasite origin and for host-initiated regulation. These models are backed by other examples in which mechanisms underlying fecundity reduction or fecundity compensation are explored. It is concluded that evolutionary theories must be supported by empirical evidence gained from studying molecular, biochemical and physiological mechanisms underlying changes in host life-history traits, ideally using organisms that have evolved together and that are in their natural environment.

Genome-wide transcriptomic profiling of Anopheles gambiae hemocytes reveals pathogen-specific signatures upon bacterial challenge and Plasmodium berghei infection

Background

The mosquito Anopheles gambiae is a major vector of human malaria. Increasing evidence indicates that blood cells (hemocytes) comprise an essential arm of the mosquito innate immune response against both bacteria and malaria parasites. To further characterize the role of hemocytes in mosquito immunity, we undertook the first genome-wide transcriptomic analyses of adult female An. gambiae hemocytes following infection by two species of bacteria and a malaria parasite.

Results

We identified 4047 genes expressed in hemocytes, using An. gambiae genome-wide microarrays. While 279 transcripts were significantly enriched in hemocytes relative to whole adult female mosquitoes, 959 transcripts exhibited immune challenge-related regulation. The global transcriptomic responses of hemocytes to challenge with different species of bacteria and/or different stages of malaria parasite infection revealed discrete, minimally overlapping, pathogen-specific signatures of infection-responsive gene expression; 105 of these represented putative immunity-related genes including anti-Plasmodium factors. Of particular interest was the specific co-regulation of various members of the Imd and JNK immune signaling pathways during malaria parasite invasion of the mosquito midgut epithelium.

Conclusion

Our genome-wide transcriptomic analysis of adult mosquito hemocytes reveals pathogen-specific signatures of gene regulation and identifies several novel candidate genes for future functional studies.

Two mosquito LRR proteins function as complement control factors in the TEP1-mediated killing of Plasmodium

Plasmodium development within Anopheles mosquitoes is a vulnerable step in the parasite transmission cycle, and targeting this step represents a promising strategy for malaria control. The thioester-containing complement-like protein TEP1 and two leucine-rich repeat (LRR) proteins, LRIM1 and APL1, have been identified as major mosquito factors that regulate parasite loads. Here, we show that LRIM1 and APL1 are required for binding of TEP1 to parasites. RNAi silencing of the LRR-encoding genes results in deposition of TEP1 on Anopheles tissues, thereby depleting TEP1 from circulation in the hemolymph and impeding its binding to Plasmodium. LRIM1 and APL1 not only stabilize circulating TEP1, they also stabilize each other prior to their interaction with TEP1. Our results indicate that three major antiparasitic factors in mosquitoes jointly function as a complement-like system in parasite killing, and they reveal a role for LRR proteins as complement control factors.

The immunogenic properties of protozoan glycosylphosphatidylinositols in the mosquito Anopheles gambiae

In contrast to humans, mosquitoes do not have an adaptive immune response to deal with pathogens, and therefore must rely on their innate immune system to deal with invaders. This facilitates the recognition of different microbes on the basis of surface components or antigens. Such antigens have been identified in various types of microbe such as bacteria and fungi, yet none has been identified in the genus protozoa, which includes pathogens such as the malaria parasite, Plasmodium falciparum and Toxoplasma gondii. This study allowed us to test the antigenic properties of protozoan glycosylphosphatidylinositol (GPI) on the mosquito immune system. We found that both P. falciparum GPI and T. gondii GPI induce the strong expression of several antimicrobial peptides following ingestion, and that as a result of the immune response against the GPIs, the number of eggs produced by the mosquito is reduced dramatically. Such effects have been associated with malaria infected mosquitoes, but never associated with a Plasmodium specific antigen. This study demonstrates that protozoan GPIs can be considered as protozoan specific immune elicitors in mosquitoes, and that P. falciparum GPI plays a critical role in the malaria parasite manipulation of the mosquito vector to facilitate its transmission.

An insect symbiosis is influenced by bacterium-specific polymorphisms in outer-membrane protein A

Beneficial bacterial symbioses are ubiquitous in nature. However, the functional and molecular basis of host tolerance to resident symbiotic microbes, in contrast to resistance to closely related bacteria that are recognized as foreign, remain largely unknown. We used the tsetse fly (Glossina morsitans), which depends on symbiotic flora for fecundity and has limited exposure to foreign microbes, to investigate the tolerance phenomenon exhibited during symbiosis. We examined the potential role of bacterium-specific polymorphisms present in the major bacterial surface protein, outer-membrane protein A (OmpA), on host infection outcomes. Tsetse were successfully superinfected with their mutualistic facultative symbiont, Sodalis glossinidius, whereas infections with Escherichia coli K12 were lethal. In contrast, tsetse were resistant to an E. coli OmpA mutant strain, whereas recombinant Sodalis expressing E. coli OmpA became pathogenic. Profiling of tsetse immunity-related gene expression incriminated peptidoglycan recognition protein (pgrp)-lb as a determinant of the infection outcomes we observed. RNAi-induced knockdown of tsetse pgrp-lb significantly reduced host mortality after infection with otherwise lethal E. coli K12. Our results show that polymorphisms in the exposed loop domains of OmpA represent a microbial adaptation that mediates host tolerance of endogenous symbiotic bacteria.

Competency of Anopheles stephensi mysorensis strain for Plasmodium vivax and the role of inhibitory carbohydrates to block its sporogonic cycle

Background

Despite the abundance of studies conducted on the role of mosquitoes in malaria transmission, the biology and interaction of Plasmodium with its insect host still holds many mysteries. This paper provides the first study to follow the sporogonic cycle of Plasmodium vivax in a wild insecticide-resistant mysorensis strain of Anopheles stephensi, a major vector of vivax malaria in south-eastern Iran. The study subsequently demonstrates that host-parasite sugar binding interactions are critical to the development of this parasite in the salivary glands of its mosquito host. The identity of the receptors or sugars involved was revealed by a receptor "pre-saturation" strategy in which sugars fed to the mosquitoes inhibited normal host-parasite interactions.

Methods

Anopheles stephensi mysorensis mosquitoes were artificially infected with P. vivax by feeding on the blood of gametocytaemic volunteers reporting to local malaria clinics in the Sistan-Baluchistan province of south-eastern Iran. In order to determine the inhibitory effect of carbohydrates on sporogonic development, vector mosquitoes were allowed to ingest blood meals containing both gametocytes and added carbohydrates. The carbohydrates tested were GlcNAc, GalNAc, arabinose, fucose, mannose, lactose, glucose and galactose. Sporogonic development was assessed by survival of the parasite at both the oocyst and sporozoite stages.

Results

Oocyst development was observed among nearly 6% of the fed control mosquitoes but the overall number of mosquitoes exhibiting sporozoite invasion of the salivary glands was 47.5% lower than the number supporting oocysts in their midgut. Of the tested carbohydrates, only arabinose and fucose slightly perturbed the development of P. vivax oocysts at the basal side of the mosquito midgut, and the remaining sugars caused no reductions in oocyst development. Strikingly however, sporozoites were completely absent from the salivary glands of mosquitoes treated with mannose, GalNAc, and lactose.

Conclusion

The study indicates that An. stephensi in southern Iran has the potential to survive long enough to be re-infected and transmit vivax malaria several times, based on the average adult female longevity (about 30 days) and its gonotrophic cycle (2–3 days) during the malaria transmission season. Certain sugar binding interactions are important for the development of P. vivax sporozoites, and this information may be instrumental for the development of transmission blocking strategies.

Insulin regulates aging and oxidative stress in Anopheles stephensi

Observations from nematodes to mammals indicate that insulin/insulin-like growth factor signaling (IIS) regulates lifespan. As in other organisms, IIS is conserved in mosquitoes and signaling occurs in multiple tissues. During bloodfeeding, mosquitoes ingest human insulin. This simple observation suggested that exogenous insulin could mimic the endogenous hormonal control of aging in mosquitoes, providing a new model to examine this phenomenon at the organismal and cellular levels. To this end, female Anopheles stephensi mosquitoes were maintained on diets containing human insulin provided daily in sucrose or three times weekly by artificial bloodmeal. Regardless of delivery route, mosquitoes provided with insulin at 1.7 x 10(-4) and 1.7 x 10(-3) micromol l(-1), doses 0.3-fold and 3.0-fold higher than non-fasting blood levels, died at a faster rate than controls. In mammals, IIS induces the synthesis of reactive oxygen species and downregulates antioxidants, events that increase oxidative stress and that have been associated with reduced lifespan. Insulin treatment of mosquito cells in vitro induced hydrogen peroxide synthesis while dietary supplementation reduced total superoxide dismutase (SOD) activity and manganese SOD activity relative to controls. The effects of insulin on mortality were reversed when diets were supplemented with manganese (III) tetrakis (4-benzoic acid) porphyrin (MnTBAP), a cell-permeable SOD mimetic agent, suggesting that insulin-induced mortality was due to oxidative stress. In addition, dietary insulin activated Akt/protein kinase B and extracellular signal-regulated kinase (ERK) in the mosquito midgut, suggesting that, as observed in Caenorhabditis elegans, the midgut may act as a 'signaling center' for mosquito aging.

Reactive oxygen species modulate Anopheles gambiae immunity against bacteria and Plasmodium

The involvement of reactive oxygen species (ROS) in mosquito immunity against bacteria and Plasmodium was investigated in the malaria vector Anopheles gambiae. Strains of An. gambiae with higher systemic levels of ROS survive a bacterial challenge better, whereas reduction of ROS by dietary administration of antioxidants significantly decreases survival, indicating that ROS are required to mount effective antibacterial responses. Expression of several ROS detoxification enzymes increases in the midgut and fat body after a blood meal. Furthermore, expression of several of these enzymes increases to even higher levels when mosquitoes are fed a Plasmodium berghei-infected meal, indicating that the oxidative stress after a blood meal is exacerbated by Plasmodium infection. Paradoxically, a complete lack of induction of catalase mRNA and lower catalase activity were observed in P. berghei-infected midguts. This suppression of midgut catalase expression is a specific response to ookinete midgut invasion and is expected to lead to higher local levels of hydrogen peroxide. Further reduction of catalase expression by double-stranded RNA-mediated gene silencing promoted parasite clearance by a lytic mechanism and reduced infection significantly. High mosquito mortality is often observed after P. berghei infection. Death appears to result in part from excess production of ROS, as mortality can be decreased by oral administration of uric acid, a strong antioxidant. We conclude that ROS modulate An. gambiae immunity and that the mosquito response to P. berghei involves a local reduction of detoxification of hydrogen peroxide in the midgut that contributes to limit Plasmodium infection through a lytic mechanism.

The parasite invasion marker SRPN6 reduces sporozoite numbers in salivary glands of Anopheles gambiae

For malaria transmission to occur, Plasmodium sporozoites must infect the salivary glands of their mosquito vectors. This study reports that Anopheles gambiae SRPN6 participates in a local salivary gland epithelial response against the rodent malaria parasite, Plasmodium berghei. We showed previously that SRPN6, an immune inducible midgut invasion marker, influences ookinete development. Here we report that SRPN6 is also specifically induced in salivary glands with the onset of sporozoite invasion. The protein is located in the basal region of epithelial cells in proximity to invading sporozoites. Knockdown of SRPN6 during the late phase of sporogony by RNAi has no effect on oocyst rupture but significantly increases the number of sporozoites present in salivary glands. Despite several differences between the passage of Plasmodium through the midgut and the salivary glands, this study identifies a striking overlap in the molecular responses of these two epithelia to parasite invasion.

Fz2 and cdc42 mediate melanization and actin polymerization but are dispensable for Plasmodium killing in the mosquito midgut

The midgut epithelium of the mosquito malaria vector Anopheles is a hostile environment for Plasmodium, with most parasites succumbing to host defenses. This study addresses morphological and ultrastructural features associated with Plasmodium berghei ookinete invasion in Anopheles gambiae midguts to define the sites and possible mechanisms of parasite killing. We show by transmission electron microscopy and immunofluorescence that the majority of ookinetes are killed in the extracellular space. Dead or dying ookinetes are surrounded by a polymerized actin zone formed within the basal cytoplasm of adjacent host epithelial cells. In refractory strain mosquitoes, we found that formation of this zone is strongly linked to prophenoloxidase activation leading to melanization. Furthermore, we identify two factors controlling both phenomena: the transmembrane receptor frizzled-2 and the guanosine triphosphate–binding protein cell division cycle 42. However, the disruption of actin polymerization and melanization by double-stranded RNA inhibition did not affect ookinete survival. Our results separate the mechanisms of parasite killing from subsequent reactions manifested by actin polymerization and prophenoloxidase activation in the A. gambiae–P. berghei model. These latter processes are reminiscent of wound healing in other organisms, and we propose that they represent a form of wound-healing response directed towards a moribund ookinete, which is perceived as damaged tissue.

A dangerous journey awaits malaria Plasmodium parasites ingested by a mosquito. Most parasites are destroyed by host responses in the midgut, and in parasite-resistant refractory strains of mosquito the mortality can reach 100%. This midgut “bottleneck” represents an appealing target for reducing malaria transmission by the genetic control of wild mosquitoes. However, the killing mechanisms are still unclear. In this study, electron microscopical analyses followed the entire midgut invasion process in mosquitoes to identify the major site(s) and ultrastructural features of Plasmodium killing. The authors found that invasion can be divided into two steps: a swift passage through a midgut cell, followed by establishment of the parasite in the basal extracellular space, where it becomes an important target for destruction by soluble immunity factors. In refractory mosquitoes, dead parasites are associated with the formation of organelle-free zones of actin in adjacent midgut cells, and melanin deposition on the parasite surface. The authors identify two genes, called frizzled-2 and cell division cycle 42, that control these phenomena. Actin zone formation and melanization are generally thought to be killing mechanisms; however, the authors show by gene silencing that neither is lethal to Plasmodium. Instead, these mechanisms may represent a form of mosquito wound-healing response that is triggered by the presence of a moribund parasite.

The insulin signaling cascade from nematodes to mammals: insights into innate immunity of Anopheles mosquitoes to malaria parasite infection

As revealed over the past 20 years, the insulin signaling cascade plays a central role in regulating immune and oxidative stress responses that affect the life spans of mammals and two model invertebrates, the nematode Caenorhabitis elegans and the fruit fly Drosophila melanogaster. In mosquitoes, insulin signaling regulates key steps in egg maturation and immunity and likely affects aging, although the latter has yet to be examined in detail. Reproduction, immunity and aging critically influence the capacity of mosquitoes to effectively transmit malaria parasites. Current work has demonstrated that molecules from the invading parasite and the blood meal elicit functional responses in female mosquitoes that are regulated through the insulin signaling pathway or by cross-talk with interacting pathways. Defining the details of these regulatory interactions presents significant challenges for future research, but will increase our understanding of mosquito/malaria parasite transmission and of the conservation of insulin signaling as a key regulatory nexus in animal biology.

Efficiency of salivary gland invasion by malaria sporozoites is controlled by rapid sporozoite destruction in the mosquito haemocoel

For successful transmission to the vertebrate host, malaria sporozoites must migrate from the mosquito midgut to the salivary glands. Here, using purified sporozoites inoculated into the mosquito haemocoel, we show that salivary gland invasion is inefficient and that sporozoites have a narrow window of opportunity for salivary gland invasion. Only 19% of sporozoites invade the salivary glands, all invasion occurs within 8h at a rate of approximately 200 sporozoites per hour, and sporozoites that fail to invade within this time rapidly die and are degraded. Then, using natural release of sporozoites from oocysts, we show that haemolymph flow through the dorsal vessel facilitates proper invasion. Most mosquitoes had low steady-state numbers of circulating sporozoites, which is remarkable given the thousands of sporozoites released per oocyst, and suggests that sporozoite degradation is a rapid immune process most efficient in regions of high haemolymph flow. Only 2% of Anopheles gambiae haemocytes phagocytized Plasmodium berghei sporozoites, a rate insufficient to explain the extent of sporozoite clearance. Greater than 95% of haemocytes phagocytized Escherichia coli or latex particles, indicating that their failure to sequester large numbers of sporozoites is not due to an inability to engage in phagocytosis. These results reveal the operation of an efficient sporozoite-killing and degradation machinery within the mosquito haemocoel, which drastically limits the numbers of infective sporozoites in the mosquito salivary glands.

Getting infectious: formation and maturation of Plasmodium sporozoites in the Anopheles vector

Research on Plasmodium sporozoite biology aims at understanding the developmental program steering the formation of mature infectious sporozoites - the transmission stage of the malaria parasite. The recent identification of genes that are vital for sporozoite egress from oocysts and subsequent targeting and transmigration of the mosquito salivary glands allows the identification of mosquito factors required for life cycle completion. Mature sporozoites appear to be equipped with the entire molecular repertoire for successful transmission and subsequent initiation of liver stage development. Innovative malaria intervention strategies that target the early, non-pathogenic phases of the life cycle will crucially depend on our insights into sporozoite biology and the underlying molecular mechanisms that lead the parasite from the mosquito midgut to the liver.

Increased melanizing activity in Anopheles gambiae does not affect development of Plasmodium falciparum

Serpins are central to the modulation of various innate immune responses in insects and are suspected to influence the outcome of malaria parasite infection in mosquito vectors. Three Anopheles gambiae serpins (SRPN1, -2, and -3) were tested for their ability to inhibit the prophenoloxidase cascade, a key regulatory process in the melanization response. Recombinant SRPN1 and -2 can bind and inhibit a heterologous phenoloxidase-activating protease and inhibit phenoloxidase activation in vitro. Using a reverse genetics approach, we studied the effect of SRPN2 on melanization in An. gambiae adult females in vivo. Depletion of SRPN2 from the mosquito hemolymph increases melanin deposition on foreign surfaces such as negatively charged Sephadex beads. As reported, the knockdown of SRPN2 adversely affects the ability of the rodent malaria parasite Plasmodium berghei to invade the midgut epithelium and develop into oocysts. Importantly, we tested whether the absence of SRPN2 from the hemolymph influences Plasmodium falciparum development. RNAi silencing of SRPN2 in an An. gambiae strain originally established from local populations in Yaoundé, Cameroon, did not influence the development of autochthonous field isolates of P. falciparum. This study suggests immune evasion strategies of the human malaria parasite and emphasizes the need to study mosquito innate immune responses toward the pathogens they transmit in natural vector-parasite combinations.

Apoptosis-like death as a feature of malaria infection in mosquitoes

Malaria parasites of the genusPlasmodiummake a hazardous journey through their mosquito vectors. The majority die in the process, many as a result of the action of mosquito defence mechanisms. The mosquito too is not unscathed by the encounter with these parasites. Tissue damage occurs as a result of mid-gut invasion and reproductive fitness is lost when many developing ovarian follicles are resorbed. Here we discuss some of the mechanisms that are involved in killing the parasite and in the self-defence mechanisms employed by the mosquito to repair the mid-gut epithelium and to manipulate resources altering the trade-off position that balances reproduction and survival. In all cases, cells die by apoptotic-like mechanisms. In the midgut cells, apoptosis-induction pathways are being elucidated, the molecules involved in apoptosis are being recognised andDrosophilahomologues sought. The death of ookinetes in the mosquito mid-gut lumen is associated with caspase-like activity and, although homologues of mammalian caspases are not present in the malaria genome, other cysteine proteases that are potential candidates have been discussed. In the ovary, apoptosis of patches of follicular epithelial cells is followed by resorption of the developing follicle and a subsequent loss of egg production in that follicle.